Why are so many people swapping this engine into their cars? To answer that you have to take a good look at the 240SX. Rear wheel drive, sophisticated multi-link independent rear suspension, near perfect front / rear weight balance give this car light, tossable, very impressive handling. The 240SX’s road race very competitively, often able to make up for lack of power in every turn, consistently catching up against some impressive rivals.
With plenty of aftermarket suspension support you can really get a world-class handling package put together for a reasonable amount of money. Considering that a used good condition 240SX can be found for under $5000, the entry price is way below and the performance potentials far exceed other similar performance options.
So the handling is all set, what about the power? Due to marketing decisions, the US never got to see the 180SX / 200SX / Silvia with the high-performance engine the car was designed to exploit, the SR20DET. Instead, we got the KA24 engines, SOHC in 89 and 90 and DOHC from 91 up.
The SR20DET engine came in the 180SX / 200SX / Silvia S13, S14 and S15 models. The transmission and rear differential are the same for all the models, so they are factory designed to withstand the power from the SR already. The engine bolts right in to the 240SX, so does the transmission. There are several things that need to be done custom, but not a lot. The electrical system wiring is not very difficult.
The SR20DET also responds very well to modifications. Stock boost can be safely increased to about 14 psi (really pushing it), the turbo can be upgraded, and JWT makes a custom injector / ECU / MAF upgrade. Not to mention the tons of mods available for the engine in Japan. In Japan, the Silvia is extremely popular so there is a huge market for modifications and parts.